Richard Smith, Joji Muramoto, Tim Hartz and Michael Cahn

UCCE Emeritus Farm Advisor, Extension Specialist, Emeritus, Extension Specialist and Irrigation and Water Resources Farm Advisor.

The winter of 2023 had the highest rainfall years in the last 25 years. The high rainfall resulted in flooding onto farmland along the main branch of the Salinas River in both January and March. The flood waters disrupted planting schedules as well as inundated established plantings resulting in a disruption to the beginning of the vegetable production season.

The river also deposited a layer of sediments in flooded fields (Photo 1). The sediments came from several sources: river sediments from as far away as San Luis Obispo County; sediments from side channels; and soil sediments scoured from upstream farms. Several growers and industry personnel have asked what is the composition of these sediments? In April after the flooding had subsided, we collected samples at river crossings from San Lucas to Salinas. The layer of sediment left by the flood waters tended to curled up as it dried out and were easy to collect. Any field soil was brushed from the bottom of the sediments and they were sent to the UC Davis Analytical Laboratory for analysis.

Tables 1 and 2 have analysis of the sediments collected. The data in the table is arranged with sites from south to north; the two side channels, Arroyo Seco and Monroe Canyon are listed separately. Monroe Canyon is the drainage that comes from the west side of Hwy 101 just south of the intersection of Hwy 101 and Central Avenue north of King City; it cuts through a large section of the Monterey shale formation that contains elevated levels of cadmium.

The San Lucas, Arroyo Seco and Monroe Canyon samples are coarser indicating that they were transported by rapid water movement, while the rest of the samples are dominated by silts and clays, indicating that they were transported by slower moving water. In general, there is a good correlation between the clay content of the sediments and nutrient and organic matter content. Higher nutrients in the silt and clay sediments include total nitrogen, calcium, magnesium, sulfate, zinc and iron. The sediments are generally fertile which may indicate that they are at least partially composed of soil eroded from farmed fields farther upstream. Sediments that are low in phosphorus likely originated from non-farmed or vineyard areas.

The elevated cadmium levels measured in sediments from the Arroyo Seco and Monroe Canyon indicate that these side channels carried sediments from the Monterey shale formation which has naturally high levels of cadmium into the Salinas River. Presumably these sediments originating in the Monterey shale formation are transported to areas further downstream by flood waters. 

Photo 1. Sediments deposited in a field along the Salinas River

Table 1. Analysis of river sediment samples from locations from San Lucas to Salinas and two side channel locations.

Table 2. Analysis of river sediment samples from locations from San Lucas to Salinas and two side channel locations.

I have received a handful of calls this season with concerns about “trash bugs,” a catch all term for various soil invertebrates. These soil invertebrates include root maggots, springtails, bulb mites, and symphylans, all which will happily feed on decomposing plant debris, i.e., trash. If given the opportunity, trash bugs will feed on seedlings and transplants, but high pressure can usually be avoided by allowing plant debris to fully breakdown before planting the next crop. A long enough pause between plant debris and seedlings acts as a sort of field-level host-free period, allowing trash bug populations to drop before planting.

Why does it feel like we have more trash bugs this season?

We had an unseasonably cool, wet spring. Under these conditions, crop debris from the fall was breaking down much slower than usual. With the compressed start to the season, folks have been eager to plant into fields as soon as possible, so many were likely planting into fields with higher-than-optimal plant debris.

By mid-April, daily temperatures were starting to get more back to normal, so why are we still dealing with more trash bugs? From talking with a handful of growers and PCAs, the slow start to the season has set back the whole planting schedule. To try and bring things back to schedule as close as possible, folks are still pushing the limit on how quickly they turn around fields for the next crop. A perfect storm for trash bug pressure.

Which trash bug am I dealing with?

A diverse group of pests can cause stunting and stand loss, so accurate pest ID is critical for successful management.

Root Maggots (seedcorn maggots or cabbage maggots, Delia spp.)

Root maggots are the larvae of small grey flies. The adult flies are commonly caught on sticky cards that are deployed for monitoring thrips and aphids.

Seedcorn maggot, Delia platura, seems to be the primary culprit that I have come across that is stunting brassicas. Seedcorn maggots tend to hit fields within a week or two after planting, causing patchy stands and stunted seedlings. Closely related cabbage maggots tend to hit fields a few weeks after planting and can continue to cause damage as plants mature.

To scout for root maggots, pull up stunted plants (see picture #1 below) and check roots for small, yellowish-white maggots (less than ¼ of an inch long). They can be easy to miss when they're all tangled up in the roots (picture #2). If scouting is delayed, you may also find the brown pupa (picture #3). With seedcorn maggots, timing a reactive control (i.e., insecticide application) can be tricky since the damage may go unnoticed until the maggots have already pupated and left the field.

From left to right: #1 broccoli stunting caused by root maggot feeding; #2 maggot tangled up in the roots of a broccoli plant; #3 root maggot pupa found next to a stunted broccoli plant.

Springtails and soil mites

Pest springtails and soil mites are trickier to identify, since non-pest species are common in healthy soils. The one species of springtail that has been identified as an occasional pest of lettuce and brassicas is Protaphorura fimata, while the main pest mites are bulb mites (Rhizoglyphus spp., Tyrophagus spp.). Both P. fimata and bulb mites are small and whiteish while non-pest species are often larger and more colorful.

Clockwise from top left; Protaphorura fimata, the springtail species that can be a pest (Photo Credit: Shimat Joseph, previous UC IPM Advisor); non-pest springtails (note that these springtails are more colorful and have long antennae); a predatory soil mite (not a pest); and two bulb mites (Photo Credit: Jack Kelly Clark, UC IPM).

Symphylans

Symphylans look like small (less than ½ an inch), white centipedes. Since symphylans are fast and mobile, you will be more likely to catch symphylans in action if you dig around stunted plants that are near healthy looking plants. If you readily find symphylans just by digging, the symphylan pressure is probably high enough to cause economic damage.

Lower symphylan densities can be harder to observe, so you may want to try bait-trapping with potato or beet wedges if you suspect symphylans but cannot find any by digging around stunted plants. The bait-trapping method can also be used to scout for springtails.

For more on the identification, scouting, and management of these pests, be sure to reference the UC IPM Pest Management Guidelines:

https://ipm.ucanr.edu/agriculture/cole-crops/

https://ipm.ucanr.edu/agriculture/lettuce/

Related blog posts from past entomology advisors:

Root maggots:

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=9804

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=4301

Symphylans:

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=18819

Springtails:

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=16769

Bulb Mites:

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=13271

Join us for Fusarium wilt of lettuce cultivar trial field day, being held on July 28!

Richard Smith, Emeritus Vegetable Crop and Weed Science Farm Advisor; Eric Brennan, Research Horticulturalist USDA, ARS; Daniel Geisseler, Plant Nutrition Specialist, UCD; and Joji Muramoto, Organic Farming Specialist, UCSC

In a recent blog on soil health, we discussed the importance of adding carbon to soils in Salinas Valley vegetable production operations to improve soil health. We grappled with the question, can additions of carbon to the soil help reduce the incidence and severity of soilborne diseases? At this point, there is not a straightforward answer to that question, but the issue of soil health as one of a series of approaches to managing soilborne disease cannot be entirely discounted. The reason is, that by increasing soil  microbial activity, soilborne pathogens may be placed at a disadvantage due to predation from or competition with beneficial soil microbes. However, good scientific evidence of that specific effect is not yet well developed for key soilborne diseases such as Fusarium and Pythium wilts of lettuce.

Whether or not improving soil health reduces soilborne diseases, it does provide other critical benefits to soils (improved tilth, water infiltration and retention, etc) which provides incentives to employ practices that keep soils healthy and thereby improve the productivity of vegetables farms. Two of the three measures of soil health suggested by the Soil Health Institute are tied directly to soil carbon: 1) total soil carbon content of soil and 2) potentially mineralizable carbon content of soil. Increasing the quantity of carbon to the soil and/or retaining carbon in the soil (by minimizing tillage), are key practices that move a soil towards improved soil health.

As will be seen in our discussion below, some sources of carbon are more long-lasting than others. The C:N ratio drives the rate of decomposition of the material. All organic materials have a rapid phase of decomposition in which the labile (i.e., easily decomposable) materials are quickly broken down in 4-6 weeks. This phase is followed by a slower, steady rate of decomposition in which the recalcitrant (i.e., decomposition resistant) materials break down over an extended period of time that is measured in months/years.

The following is a brief discussion of the sources of carbon for soils in Salinas Valley vegetable production operations.

Cash Crop Residues and Root Exudates: Cash crops provide carbon to the soil when crop residues are incorporated after harvest and through root exudates while the crop is growing. Table 1 shows that cole crops such as broccoli and Brussels sprouts return the highest amount of residue and carbon to the soil of the commonly grown vegetables. They are followed by celery, lettuce and then spinach. This biomass and carbon data are average values from measurements of crops in commercial production fields. There is significant variability in the data due to differences from field to field, but these average values provide a reasonable estimate of the amount of carbon returned to the soil when crops are terminated. The high concentration of nitrogen in vegetable crop tissue drives rapid decomposition of a large portion of the residual biomass in the first four to six weeks after incorporation. The higher the percent nitrogen in the tissue, the higher the initial rate of mineralization of the residue (Figure 1). However, after the initial phase of decomposition of the labile carbon fraction, the rate of decomposition significantly declines which provides a longer-term source of carbon to the soil. The main vegetable rotations in the Salinas Valley, such as lettuce and broccoli, provide moderate to high amounts of carbon to the soil.  Although a significant amount of carbon is mineralized due to the high nitrogen content of the residue, a significant amount of long-term carbon remains in the soil. In addition to crop residues, crop plants add considerable amounts of carbon to the soil in root biomass and as exudates. For example, the long-term, organic systems study at the USDA-ARS in Salinas found that over 8 years with 8 crops of lettuce and 6 crops of broccoli, roughly a third of the carbon inputs from the vegetables came from root biomass and root exudates, with more than half of the below ground inputs coming from root exudates (White et al. 2020). Root exudates are being researched for their role in stimulating soil microbes and suppressing soilborne pathogens. This is clearly an important area of research that needs more attention.

Table 1. Sources of carbon for Salinas Valley soils

1 – estimate; 2 – at 10 lbs per gallon; 3 – example from Dong et al, 2019. 4. Cover crop data, other than for white mustard, is based on Brennan and Boyd 2012, and Brennan et al. 2013.

Figure 1. Mineralization of vegetable crop residues with different concentrations of N in the tissue. Source: Hartz 2020.

Cover Crop Residues and Root Exudates: Biomass and carbon inputs to the soil from cover crops also vary widely depending on the planting date, variety, growing conditions, etc. Biomass and carbon additions by cover crops was similar to the higher producing vegetable crops. However, the percent nitrogen and resulting C:N ratio of the residue varies depending on when the crop is terminated and incorporated into the soil. Cover crops that are allowed to mature have lower nitrogen and greater carbon contents, as well as contain more biomass (Table 2). To obtain the greatest quantity of carbon from the cover crop, it is best to allow cover crops to mature as long as possible given all the practical constraints such as: availability of equipment, weather conditions, scheduled planting the subsequent cash crop and the risk of seed set by the cover crop. For more information about the risk of seed set by cover crops see: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=55520. Unfortunately, due to the issues with high land rents and crop production scheduling issues, cover crops are used on only about 5% of vegetable production ground. The nitrogen scavenging credits  for over-wintered cover crops in Ag Order 4.0 will hopefully increase their use; this video discusses these credits. Also, creative use of cover crops such as fall-grown cover crops (Aug-Sept to Oct-Nov) may also increase the use of cover crops.  

The on-going, long-term organic cropping systems study at the USDA-ARS in Salinas estimated that carbon addition from cover crop roots were about 18% of the carbon added by the cover crop shoots, and that carbon from cover crop root exudates was slightly more than half (about 64%) of the carbon in cover crop root biomass. So if we assume that shoot and root biomass is about 42% carbon, a cover crop that produces 4500 lbs/acre of oven dry shoot biomass (the minimum required in the Ag. Order 4.0 regulation) would add approximately the following amounts of carbon to the soil from shoot biomass (1890 lbs carbon/acre), root biomass (340 lbs carbon/acre), and root exudates (218 lbs carbon/acre), or 2448 lbs carbon/acre overall.  These estimates are based on data from this this recent paper (White at al. 2020). When carbon is added to the soil as crop residue, it is essentially adding energy to the soil that helps to fuel the biomass and the diversity of organisms in the soil food web.  You can estimate the amount of oven-dry shoot biomass that a cereal cover crop adds to the soil by measuring the length of main stem as described in this video (A Simple Method to Help Farmers Estimate Cereal Cover Crop Shoot Biomass).

Table 2. Cover crop biomass and carbon accumulation over 3 weeks

Compost and manure: The use of manure and compost in vegetable production has declined in recent years due to food safety concerns. However, the use of compost is one of easiest and most cost-effective ways to increase the quantity of carbon in soils. In the long-term organic systems study at the USDA-ARS in Salinas the yearly application of compost increased total soil carbon (Figure 2).

Many produce buyers restrict the use of compost due to food safety concerns, but ironically, many of these same buyers encourage growers to employ sustainable production practices. Clearly, there is a disconnect between these two influences exerted by the buyers. Recently, both large individual processors and the Leafy Greens Marketing Agreement have focused attention on substantially improving confidence in the safety of compost. A systems-based approach between the leafy greens growers/handlers and a few engaged composters, distributors, and hauler/spreaders have been conducted to identify key knowledge gaps, assess the need for new validation studies, and to establish an enhanced verification system of process control requirements. Given the importance of adding carbon into Salinas Valley soils, it is important to encourage the use of compost if food safety concerns can be effectively managed.

Figure 2. Data shows levels of soil carbon in soil in treatment  plots with yearly compost application versus no compost addition. Cover crops were grown every 4 years and both treatments received the same inputs of organic fertilizers. Crosses show the average of all data points within each treatment . Source: Figure 6 in White et al. 2020.

Organic  Fertilizers: As shown in Table 1, organic fertilizers contain substantial amounts of carbon. The amount of carbon applied in typical application scenarios used by growers indicates that high amounts of carbon are also added with the nitrogen and other nutrients that they contain. The C:N ratio of dry organic fertilizers is generally low and from 40 to 60% of material decomposes in the first twelve weeks after application (Figure 3). However, a significant amount of the carbon in these materials is recalcitrant and remains in the soil as a long-term source (Hartz et al, 2000). In a recent study comparing 20 paired organic vs conventional farms where the main difference in carbon inputs was organic fertilizer, found higher microbial biomass in the soil of the organic farms (Smith et al, 2022).

The percent carbon in liquid organic fertilizers can vary from 15 to 25% (Table 1). For instance, if a material weighs 10 lbs/gallon that would be equivalent to 1.5 to 2.5 lbs carbon per gallon of material. Little is known about the mineralization characteristics of carbon in liquid fertilizers, but it makes sense that it may be fairly labile, if for no other reason, because the particle size of the material is small and readily accessible to microbes.

Figure 3. Percent mineralization of nitrogen from yard waste compost and organic fertilizers in 12 weeks (the nitrogen data is used as a proxy for the proportion of mineralization of carbon from these materials).Source: Lazicki et al, 2020.

Humic Acid Materials: We recently evaluated dry and liquid humic acid materials. The dry humic material contained 38% carbon and the liquid material contained 3.5% (if the material weighs 10 lbs/gallon = 0.35 lbs carbon/gallon). Clearly, these materials are a modest source of carbon, given the amount of these materials typically added to the soil.

Biochar: Is a charcoal-like source of carbon that can be made from a variety of sources of biomass at high temperatures in an oxygen limited environment (pyrolysis). The resulting material has properties that are different than biomass that has broken down as the result of microbial decomposition. Depending on the feedstocks and temperatures used, the material can vary in surface area, porosity, ash content and C:N ratio.  One study measured that the carbon content of biochar was about 50% (Dong et al. 2019). There is a great deal of research being conducted on biochar to better understand the benefits that it provides depending on its specific characteristics. Sanjai Parikh at UCD gave a nice summary of biochar in this presentation: https://www.cdfa.ca.gov/is/ffldrs/frep/pdfs/Biochar/ParikhFREPBiocharPresentation.pdf

For the purposes of this article, we will not delve deeply into this material, but suffice to say that it provides a unique source of carbon that does not necessarily feed the microbial community, but does play other roles in the soil.

In summary: Additions of carbon to the soil provide a key practice for improving soil health. Crop residues provide the most constant and consistent source of carbon for Salinas Valley soils. However, growers can augment carbon additions to soils by growing cover crops and applying compost. Organic growers benefit from the use of organic fertilizers which also contain significant quantities of carbon. Compost is a concentrated source of organic matter that was shown to have long-lasting effects by increasing levels of carbon in the soil.

References:

Brennan E.B., N.S. Boyd. 2012. Winter cover crop seeding rate and variety affects during eight years of organic vegetables: I. Cover crop biomass production. Agronomy Journal 104:684-698.

Brennan E.B., N.S. Boyd, R.F. Smith. 2013. Winter cover crop seeding rate and variety affects during eight years of organic vegetables: III. Cover crop residue quality and nitrogen mineralization. Agronomy Journal 105171-182.

Dong, X., B.  Singh, G. Li, Q. Lin, X. Zhao. 2019. Biochar increased field soil inorganic carbon content five years after application. Soil and Tillage Research 186; 36-41.

Hartz, T.K., Mitchell, J.P., Giannini, C., 2000. Nitrogen and carbon mineralization dynamics of manures and composts. Hortscience 35, 209-212.

Hartz T.K., T.G. Bottoms. 2010. Humic Substances Generally Ineffective in Improving Vegetable Crop Nutrient Uptake or Productivity. Hortscience 45:906-910.v

Hartz, T.K. 2020. Efficient Nutrient Management in California Vegetable Production, UC Agriculture and Natural Resources Publishing, Publication 3555. pp. 122.

Lazicki, P., Geisseler, D., Lloyd, M., 2020. Nitrogen mineralization from organic amendments is variable but predictable. Journal of Environmental Quality, DOI: 10.1002/jeq1002.20030, available at https://acsess.onlinelibrary.wiley.com/doi/full/10.1002/jeq2.20030

Smith, R.F., M. Cahn, T. Hartz, D. Geisseler and P. Love. 2022. Nitrogen and phosphorus management of organic cool-season leafy green vegetables on large-scale farms on the Central Coast.  California Agriculture 72(2):77-84.

            https://calag.ucanr.edu/archive/?type=pdf&article=ca.2022a0010

White K.E., E.B. Brennan, M.A. Cavigelli, R.F. Smith. 2020. Winter cover crops increase readily decomposable soil carbon, but compost drives total soil carbon during eight years of intensive, organic vegetable production in California. PLoS One 15:e0228677.